Base petrochemicals such as olefins are largely produced in steam cracking plants using saturated aliphatic hydrocarbon feedstocks, such as ethane, propane, or higher molecular weight hydrocarbon mixtures such as naphtha, atmospheric and/or vacuum gas oils, and the like, at high temperatures (e.g. >800 C) in the presence of steam to crack the saturated hydrocarbon molecules to lower molecular weight unsaturated hydrocarbons such as ethylene predominately, followed by propylene, and then various quantities of C4, C5 and C6 mono- and diolefinic hydrocarbons, with a lesser quantity of C7 and higher weight saturated and unsaturated aliphatic, cyclic and aromatic hydrocarbons.
In steam cracker plants, it is common to remove acid gas components, such as carbon dioxide (CO2) and hydrogen sulfide (H2S), from hydrocarbon gas streams by intimate contact with an aqueous solution of a strong base such as sodium hydroxide (NaOH), typically referred to as a caustic solution. By reaction with the caustic contained in solution, i.e. NaOH, acid gas components such as hydrogen sulfide and carbon dioxide are converted into sodium sulfide (Na2S), sodium hydrosulfide (NaHS), sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) which are absorbed into the caustic solution and, thus, removing these acid gases from the hydrocarbon stream.
During steam cracking any sulfur containing compounds added and/or present in the hydrocarbon feed stream may be converted into hydrogen sulfide and/or organically bound sulfur compounds, and also, a content of carbon dioxide may be generated by a water-gas shift reaction. The resultant gas mixture from steam cracking may then be quenched from a temperature of about 700-1100° to a lower temperature of about 35 to 40° C. whereupon the major portion of its water and C7+ hydrocarbon content may be condensed and separated from the gas mixture. After quenching, the remaining constituents of the gas mixture may be conditioned by various steps of gas compression and refrigerative cooling to prepare it for cryogenic distillation whereby its ethylene, propylene and butenes contents will ultimately be recovered in essentially pure form for ultimate use as monomers in the production of various polymers, such as polyethylene, ethylene copolymers, polypropylene and the like.
One step required to properly condition the gas mixture prior to its cryogenic distillation is to scrub the cracked gas essentially free of any acid gas components, such as hydrogen sulfide and carbon dioxide. This is accomplished at some interstage location of a multi-stage gas compression system and, on occasion post-compression, wherein the cracked gas stream is at a pressure of from about 10 to about 20 atmospheres (atm) by contacting the compressed gas stream with an aqueous sodium hydroxide solution by countercurrent contact in a gas-liquid contact vessel often referred to as an “absorber,” “Caustic Scrubber” or “Caustic Tower.” After such gas scrubbing contact the aqueous sodium hydroxide solution which is discharged from the bottom of this tower contains, in addition to some unreacted sodium hydroxide, sodium sulfide, sodium hydrosulfide, sodium carbonate and sodium bicarbonate that results from the removal of acid gas compounds from the so scrubbed gas stream. To prevent a build-up of the concentration of these components in the caustic tower and to provide for hydraulic room to add a quantity of fresh higher strength caustic solution to the caustic tower to make up for the consumption of caustic therein, a quantity of this weak or “tower spent” caustic solution is bled away from being recirculated back to the tower. However, to maintain a proper liquid volume of caustic solution circulation within the tower, a portion of this weak or “tower spent” caustic solution is recirculated back to the tower. That quantity of weak or “spent” caustic solution which is bled away from the tower has been referred to in this art as “spent caustic.” Such tower spent caustic has to be conditioned by further processing steps in a spent caustic treatment unit to condition it as environmentally acceptable by industry standards.
The caustic tower can be an important factor in determining the production capacity of an ethylene production unit because it is necessary to remove acid gas components so that the scrubbed hydrocarbon gas has a minimal, acceptable level of these components. Further, since the majority of the cracked gas must pass through the caustic tower, if the caustic tower reaches its capacity limit, then the caustic tower can establish an equipment limit for ethylene production. Thus, the capacity of the caustic tower can be an important factor in determining the capacity of the ethylene production unit, regardless whether the ethylene production unit is being designed for grassroots construction or is an already existing facility.
Steam cracking of hydrocarbons also produces small quantities of oxygenated compounds, carboxylic acids, phenols and carbonyls, mainly acetaldehyde. Carboxylic acids and phenols are removed in the quench water tower due to their high solubility in the aqueous phase.
In an acid gas removal system, amine absorber and caustic tower, some of the oxygenated compounds are also removed. It is known in the art of hydrocarbon processing that some of these oxygenated compounds, especially carbonyl compounds and particularly acetaldehyde, will undergo polymerization in the presence of a strong base such as caustic solution. When removing acid gases with amine or caustic, aldehydes are trapped. The aldehydes in the caustic solutions reacts producing polyaldols by Aldol Condensation Reaction(s). These polymers, known in the industry as “red oil”, induce fouling of the amine absorber and/or the caustic scrubber. Aldol Condensation Reactions result the liquid red oil formation, which is a reaction product of few numbers of aldehyde monomer, and further polymerization leads to the formation of high molecular weight red/yellow solid polymer. In the AGR system, the acetaldehyde polymer will settle on internal equipment surfaces leading to fouling and eventual plugging. Fouling and plugging of the internal equipment means the unit must be shut down to perform cleaning. Every time a unit operation has to be shut down for cleaning it means that a cost is incurred due to lost production, over and above, the actual cost to clean the equipment.
Steam crackers also produce smaller quantities of light aromatics (benzene, toluene, and xylene) that are found to help dissolve and disperse the Aldol polymer formed. And it became an industry practice to introduce a solvent, advantageously benzene or toluene or xylenes, in the caustic scrubber and/or in the alkaline solution fed to the scrubber to reduce the formation of the fouling deposits by reducing red oil formation and not only by the dissolution of them.
Also, during the production of ethylene and propylene with oxygenated feedstock, such as Methanol to Olefins (MTO) and alcohol dehydration, aldehydes and carbon dioxide are produced. The amount of aldehydes produced in these processes is very high compared to the steam cracker. The other characteristic of these processes is that very low quantities of aromatics such as benzene are produced. As the concentration of aldehydes is very high, the fouling potential is excessive. In the caustic scrubber operating with the effluent of a steam cracker the presence of aromatics helps to reduce red oil fouling. On the contrary in the caustic scrubber operating with the effluent of MTO or alcohol dehydration, there are two drawbacks:    (i) There are more aldehydes, as a consequence the red oil may be increased,    (ii) There are much less aromatic byproducts, as a consequence the red oil may not be dissolved and the fouling may increase.
The capacity of a caustic tower can be, and frequently is, adversely affected by polymer formation. The cracked gas contains highly reactive carbonyls and diolefins, which can form polymers, and thus are of concern throughout the plant, but particularly in the caustic tower. The highly reactive carbonyls and diolefins, and possibly other compounds, react or polymerize to form polymers which coat, foul and plug the internals of equipment, such as the caustic tower, which reduces the equipment's efficiency and capacity and, at times, necessitates a shutdown of the equipment for cleaning. Polymer formation in the caustic tower thus reduces its capacity both by reducing its operating efficiency and by necessitating the shutdown of the caustic tower for cleaning and removing deposits of polymeric material.
The spent caustic solution leaving the caustic tower contains, in addition to sodium hydroxide, sodium sulfide, sodium hydrosulfide, sodium carbonate and sodium bicarbonate that results from the removal of acid gas compounds from the so scrubbed gas stream and also a significant content of dissolved mono- and di-olefinic hydrocarbons as well as carbonyls and other organic contaminants. In this condition, the spent caustic solution presents various problems with respect to either its environmental disposal or to its reconditioning for subsequent uses. For example, polymers tend to form in the spent caustic solution as long as the solution contains dissolved polymer precursors at an elevated temperature. Aldol condensation of dissolved oxygenated hydrocarbons (carbonyls, such as aldehydes and ketones) produces red oil polymeric products, which is and remains partially soluble in a spent caustic solution that issues from the caustic scrubbing tower. Certain highly unsaturated hydrocarbons in the cracked gas, such as acetylenes and dienes (diolefinic hydrocarbons), that pass into the spent caustic solution in the scrubbing tower may polymerize to various degrees. Liquid red oil formation, which is a product of aldol condensation of few numbers of aldehyde monomer, and further polymerization leads to the formation of high molecular weight red solid polymer. The insoluble polymeric species in the spent caustic solution precipitate out of solution and may be removed in a deoiling drum. In any event, the spent caustic solution removed from the gas scrubbing tower, even following a deoiling drum treatment, includes in dissolved form a content of such condensation and addition types of polymer and polymeric species which may later precipitate from the spent caustic solution as foulants on equipment surfaces when subsequently exposed to the spent caustic solution.
From a disposal standpoint, sodium sulfide, sodium hydrosulfide contaminants as well as the dissolved hydrocarbon and other organic contaminants impart to the spent caustic solution too high a chemical oxygen demand (COD) and/or biological oxygen demand (BOD) to allow for its environmentally acceptable disposal. Further, the alkaline value of the spent caustic stream is not useable for other purposes due to the presence therein of these contaminant components. From either perspective, the constituents of the spent caustic solution that are other than sodium hydroxide and water are contaminants which either renders it unusable or disposable absent any other further treatment.